The present invention relates to cathode ray tube (CRT) alpha-numeric displays and more particularly to preventing of drift of alpha-numeric characters on a CRT screen.
It is a requirement of a high resolution display system that the alpha-numeric characters do not undergo short term or long term drift in position on the CRT screen. This drift is caused by variations in delay between the horizontal sync pulse and the occurrence of the flyback interval in the horizontal deflection circuit. The duration of the flyback interval is very stable since it is determined by a stable LC resonant circuit consisting of the deflection yoke and a flyback capacitor that shunts the deflection yoke. The same cannot be said of the initiation of the flyback interval which occurs at the turning off of the horizontal yoke driver. Variations in the initiation time of a flyback interval can be caused by change in circuit or saturation delays caused by excitation voltage changes and temperature changes. In a fixed frequency scan system such as television, these variations in initiation of the flyback interval are usually controlled by a phase-locked-loop circuit. These phase-locked-loop circuits are frequency dependent and therefore impractical in a variable format alpha-numeric display system with a variable frequency scan.
U.S. Patent No. 4,063,133 discloses a frequency dependent horizontal deflection circuit which correct for variations in retrace initiation which are not adjusted by synchronization circuits.
U.S. Patent No. 4,263,615 describes an open loop circuit which synchronizes the horizontal drive sweep with a range of input sync pulse widths.
Therefore, other means need be provided to assure fixed detail between the start of the horizontal sync pulse and the initiation of the flyback interval.